The multiplication of animal cells in vitro, requires the presence of macromolecular growth factors in the culture medium. Serum has normally been the source of these multiplication-stimulating factors. Due to the complexities of serum, the mechanism by which these factors stimulate the growth of cells remains obscure. In these proposed studies, a growth factor called Multiplication-Stimulating Activity (MSA) will be purified and used to investiiate various parameters of cell growth regulation. MSA is produced by a line of rat liver cells which grow in the complete absence of serum. Conditioned medium from these cells is used as the source for the purification of MSA. Purified MSA is a protein of about 10,000 MW which will stimulate DNA synthesis and growth of chicken embryo fibroblasts, and has non-suppressible insulin-like activity (NSILA) and resembles the somatomedin family. Various aspects of growth control will be investigated using purified MSA, such as regulation of the cell cycle in normal and virus-transformed chicken embryo fibroblasts. In addition, parameters of membrane function and other biochemical events will be analyzed as to their role in the regulation of cell proliferation.